(1) Field of the Invention
This invention relates to an image display apparatus which uses a display panel, such as a plasma display panel, that displays images by a multi-level gray scale by dividing one TV field of the image into a plurality of subfields, and especially to an image display apparatus which displays images of improved quality. This invention also relates to an image evaluation apparatus which evaluates images displayed in such an image display apparatus.
(2) Description of the Prior Art
Image display apparatuses which use display panels based on a binary illumination system, namely, two illumination states in which each pixel can be ON or OFF, represented in this specification by plasma display panels (hereinafter simply referred to as "PDPs"), achieve gray scale displays by methods such as the Address Display Period Separated Sub-Field method. In this method, an image is displayed by dividing the time in one TV field into a plurality of subfields which are each composed of an addressing period in which ON/OFF data is written for each line of a PDP screen and a discharge sustaining period in which predetermined pixels are illuminated all at once.
It is conventionally known that when displaying a moving image in a multi-level gray scale by dividing each TV field of the moving image into a plurality of subfields, gray scale disturbance in the form of "false edge" appears on the screen.
The following is an explanation of an occurrence of such false edges when displaying a moving image. FIG. 35 shows movement of a picture pattern PA1 on a screen of a PDP 240, the picture pattern PA1 being composed of two pairs of adjacent pixels having the similar gray scale levels 127 and 128 respectively. In this example, the picture pattern PA1 moves horizontally by two pixels per TV field. In FIG. 36, the horizontal axis shows a relative position of each pixel on the screen, and the vertical axis shows a period which for convenience's sake corresponds to one TV field. FIG. 36 also shows how the movement of the picture pattern PA1 appears to a viewer. Here, a case is explained in which each piece of 8-bit data indicating one out of 256 gray scale levels is converted into a piece of 8-bit data showing ON/OFF states of eight subfields 1-8. The gray scale display is achieved in accordance with the ON/OFF states of the eight subfields. As a specific example, the time in one TV field is divided into subfields 1-8 which are assigned luminance weights, 1, 2, 4, 8, 16, 32, 64 and 128, respectively (in ascending order). In this case, the gray scale level 127 can be expressed by illuminating the subfields 1-7 (diagonally shaded areas on the left in FIG. 36) and not illuminating the subfield 8, while the gray scale level 128 can be expressed by not illuminating the subfields 1-7 and illuminating the subfield 8 (diagonally shaded area on the right in FIG. 36).
When displaying a still picture, the average luminance of one TV field of the observed image is expressed by the integral of the illumination periods between A-A' in FIG. 35, so that the desired gray scale level is properly displayed. On the other hand, when displaying a moving image, an integral of the illumination periods of either B-B' or C-C', depending on the direction followed by the eye, is observed on the retina. The total luminance weights assigned to each bit (subfield) between B-B' is approximately 0, while the total luminance weights assigned to each bit (subfield) between C-C' is approximately 255. Thus, when observing the movement of a picture pattern in which two similar gray scale levels, such as the gray scale levels 127 and 128, are adjacent, the boundary between the adjacent pixels of the gray scale levels appear profoundly disturbed as shown in FIG. 36.
As explained above, a halftone is represented by an integral of luminance values of each subfield in a time series. Accordingly, when the eye follows a moving image, luminance weights assigned to the subfields of different gray scale levels are integrated due to the position change. As a result, the halftone display appears profoundly disturbed. It should be noted here that this halftone disturbance appears as false edges in the image, and so generally referred to as the "moving image false edge." Such false occurrences in a moving image display are explained in detail in Hiraki Uchiike and Shigeru Mikoshiba, All About Plasma Display, Kogyo Chosakai Shuppan, (May 1, 1997), pp. 165-177.
In order to eliminate moving image false edges and reduce halftone disturbance in a moving image display, an attempt has been made with conventional image display apparatuses to divide the luminance weights of the subfields 7 and 8 as upper bits and intersperse the divided parts in the first and second halves of one TV field. FIG. 37 shows a subfield construction in a conventional method for reducing the moving image false edges by using ten subfields to display 8-bit gray scale levels, that is, 256 gray scale levels. The ten subfields are assigned luminance weights of 48, 48, 1, 2, 4, 8, 16, 32, 48, and 48 in order of time. That is to say, the combined luminance weight value of 64 and 128 for subfields 7 and 8 out of the eight subfields described above is divided into four equal luminance weights ((64+128)/4-192/4=48), which are then interspersed in the first and second halves of one field to prevent the occurrence of the halftone disturbance by reducing the luminance weights of the subfields of upper bits. With this technique, halftone disturbance is scarcely observed at the boundary between the adjacent pixels of gray scale levels 127 and 128 described above, so that the occurrence of the moving image false edges can be prevented for those values. However, for a different example, like the two pairs of adjacent pixels having gray scale levels 63 and 64 respectively shown in FIG. 37, halftone disturbance is inevitably observed at the boundary between the adjacent pixels. In the drawing, in the pixels of gray scale level 64, a subfield with a large luminance weight (here, the subfield 9) is turned ON while subfields with small luminance weights (here, the subfields 3, 4, 5, 6, and 8) are turned OFF. The distribution of ON/OFF subfields greatly changes from the previous pixels. As a result, halftone disturbance is inevitably observed at the boundary between the adjacent pixels. As shown in FIG. 37, the total luminance weights assigned to each bit (subfield) observed in the direction of the dotted line arrow Ya is approximately 79, while the total luminance weights assigned to each bit (subfield) observed in the direction of the dotted line arrow Yb is approximately 32. Thus, it is still not possible to prevent the occurrence of the moving image false edges in such a case.
Also, in the above-described method of evaluating the moving image false edge, the following problems are found. That is, in this method, all the luminance weights of the subfields on the dotted line arrow Ya or Yb shown in FIG. 37 are added up to detect the occurrence of the moving image false edge. In such a case, there is a possibility that the total luminance weights assigned to the subfields greatly change when the directions of the dotted line arrows slightly change due to the change of subfields included in these lines, as shown in the dotted line arrow Yc for Ya and Yd for Yb in the drawing. As described above, in the conventional method in which the luminance weights are added up based on a binary determination of ON/OFF subfields on a dotted line, only a slight change in the direction followed by the eye may generate a great difference in the result value of the total luminance weights which is used for the evaluation of the moving image false edge. This leads to a difference between the evaluation result and the actual image observed by a viewer.
Also, another problem of the conventional evaluation method is that the method does not cover slant movements of the image. That is, the conventional method responds only to the horizontal or vertical movement of the image.
Currently, the CRT displays are widely used as television displays. The CRT displays have been used as displays by a lot of users through a long period and the production cost is relatively low. Also, the CRT displays are highly evaluated for the quality of the luminance, contrast, etc. On the other hand, the CRT displays have disadvantages such as the largeness in their size or weight. These disadvantages hinder the use of the CRT displays as, for example, flat television sets which can be hung on a wall. PDPs or Liquid Crystal Displays (LCDs) have been in the spotlight as thin, light-weight displays. Recently, the LCDs, recognized to be suitable for small displays, have widely been used for note-type personal computers. However, it is still difficult to produce large LCDs. Also, another problem with the LCDs is that the display response characteristic is not satisfactory in displaying moving images, which leads to the occurrence of afterimages and the like. Compared to the LCDs, it is anticipated that the PDPs will be used as wall televisions in future since the PDPs are relatively suitable for large size.
In the usual CRT displays, when one electric beam hits one pixel, the pixel illuminates. At the same time, surrounding pixels also illuminate at a considerable level. This leads to the occurrence of a diffusion of image display and to the degradation of the spatial frequency characteristic. As to these problems, the PDPs and LCDs are given high evaluation that the image is clear. This is because the PDPs and LCDs are matrix-type displays which have a separate electrode for each pixel, helping the displays maintain independence of each piece of image display information for each pixel. However, the LCDs have a problem of afterimages due to the unsatisfactory display response characteristic, as described earlier. In contrast, the PDPs do not have the problem. As a result, the PDPs are highly appreciated as high-quality image display apparatuses all things considered.
Incidentally, in the conventional image display apparatuses using the PDPs, the same signal source and signal processing as the conventional CRT displays are used for the parts of the apparatuses other than the PDPs. As a result, noises included in the input image signals, especially two-dimensional high frequency component noises, are remarkable in the conventional PDP apparatuses, while such noises are not remarkable in the conventional CRT displays. The noises are marked especially in still pictures since they are minute.